Experience shows that, for example in connection with repairs and service of a nuclear reactor, debris may enter and then move with the water which circulates through the reactor core. The debris may, inter alia, consist of metal shavings (borings or turnings) formed in connection with the repair of, for example, a steam separator, pieces of metal wire, or other foreign particles, which have entered the system from the outside. The debris may give rise to abrasion damage, which may have serious consequences if they occur on parts which are particularly easily damaged, such as fuel rods. This may be the case if the debris adheres to a spacer.
To avoid damage of the above-mentioned kind in pressurized-water reactors, it is known to form the bottom tie plates, already existing under fuel rod bundles, with a large number of bored holes in order for each bottom tie plate, besides its normal function, to function as a debris catcher in the form of a strainer and prevent debris from reaching the fuel rod bundle with control rod guide tubes and spacers.
A type of bottom tie plate which is also intended to serve as a debris catcher is described in Swedish patent application 9100402-8. This bottom tie plate is provided with horizontal channels which cross vertical flow channels, and in at least some of the horizontal channels spiral springs with horizontal symmetry axis are arranged. The disadvantage of this solution is that it is difficult to avoid that the flow resistance across the bottom tie plate becomes too high.
A type of separate debris catcher arranged below the bottom tie plate is described in Swedish patent 465 644. This debris catcher comprises several turns of a wound tape or several concentric rings with spacing elements arranged between the turns of the tape or the rings, which spacing elements maintain a mutual distance between the turns of the tape or the rings in the radial direction. The problem with hitherto known separate debris catchers is that they provide a impermissibly high flow resistance and/or are complicated and expensive to manufacture.